Ultraviolet-curable resin composition for optical discs

ABSTRACT

There is provided an ultraviolet curing resin composition for an optical disk such as to improve the light resistance of reflective thin films made of silver or silver alloy and protect them against corroding and perform a high adhesive strength, in the optical disks. The composition comprises (1) a (meth)acrylate compound having a number average molecular weight of 700 to 3000 derived from epoxy resin, (2) a (meth)acrylate monomer having one or more of unsaturated ethylene groups, and (3) a polymerization initiator, such as 2,2-dimethoxy-2-phenylacetophenone, benzoylethyl ether and benzoylisobutyl ether, represented in the following general formula (I). Also, a tetrazole derivative may be additionally mixed into the composition.

TECHNICAL FIELD

The present invention relates to an ultraviolet curing resin compositionfor optical disks.

BACKGROUND ART

An optical disk such as CD, CD-R and DVD records information on atransparent substrate thereof such as polycarbonate by pits and landsand forms an information recording layer by providing a reflective filmthat allows laser beam to bounce off of the surface of pits and landsfor reading out the information so as to have one or a plurality of suchinformation recording layers. In optical disks, aluminum, gold, silicon,silicon compounds such as SiC and SiN, or the like have beenconventionally formed into thin films by sputtering for being used asthe reflective film. Also, in an optical disk of a type such as to havea plurality of information recording layers and read out information inthese information recording layers from one side of the optical disk, atleast one of reflective layers is formed as a translucent reflectivefilm.

On the other hand, in the optical disk, an ultraviolet curing resincomposition is used as a role of protecting the information recordinglayers for the purpose of performing surface protective coating on theabove-mentioned reflective film. Also, in an optical disk of a higherrecording density type such that two sheets of substrates are stuck toeach other on backsides thereof, namely, so that transparent substratesthereof face outward, an ultraviolet curing resin composition is usedfor sticking the substrates together.

In the above-mentioned reflective film, thin films made of gold, siliconand silicon compounds have been used for translucent films of DVDrequiring the compatibility between high transmittance and reflectance,the problem is that gold costs high while silicon and silicon compoundsare troublesome to handle due to easy breaking and cracking and areoxidized in being formed into films so as to contaminate a substratesurface of an optical disk. Accordingly, the use of thin films made ofsilver or silver alloy in which a slight quantity of gold, palladium,copper and the like are added to silver has been studied as substitutesfor the above-mentioned thin films. For example, a reflective film foroptical disks comprising silver alloy comprising silver as a majorcomponent is disclosed in U.S. Pat. No. 6,007,889.

Silver or the above-mentioned silver alloy, however, has a problem ofbeing essentially inferior in light resistance and not having asufficient durability due to easy corroding under the conditions of hightemperature and high humidity, so that thin films made of silver or thesilver alloy are used with difficulty as a reflective film for formingan information recording layer. In order to improve this, for example, atechnique is disclosed in Japanese Unexamined Patent Publication No.2001-167478 for maintaining initial properties by using an adhesivecomprising an ultraviolet curing type composition with low waterabsorption and low moisture permeability for sticking the substratestogether.

Conventional techniques, however, offer no solution to a problem suchthat silver and silver alloy are inferior in light resistance and theyalso do not necessarily have sufficient adhesive force.

In view of the above-mentioned existing situation, the present inventionis intended for providing an ultraviolet curing resin composition suchas to improve the light resistance of thin films made of silver orsilver alloy and protect them against corroding and perform a highadhesive strength, in an optical disk in which an information recordinglayer is formed by the thin films made of silver or silver alloy.

DISCLOSURE OF THE INVENTION

The present invention is an ultraviolet curing resin composition for anoptical disk in which an information recording layer is formed by thinfilms made of silver or alloy comprising silver as a major component,the ultraviolet curing resin composition for an optical disk comprising(1) a (meth)acrylate compound having a number average molecular weightof 700 to 3000 derived from epoxy resin, (2) a (meth)acrylate monomerhaving one or more of unsaturated ethylene groups, and (3) apolymerization initiator represented in the following general formula(I).

In the formula, R¹ denotes an alkyl group with a carbon number of 1 to4. R² denotes a hydrogen atom or an alkyl group with a carbon number of1 to 4. R³ denotes a phenyl group or an alkyl group with a carbon numberof 1 to 4.

In the present invention, the above-mentioned (meth)acrylate compound(1) having a number average molecular weight of 700 to 3000 derived fromepoxy resin may be an adduct of ε-caprolactone-modified (meth)acrylateto epichlorohydrin-bisphenol A type (epi-bis type) epoxy resin. Also, inthe present invention, a tetrazole derivative may be additionally mixed.The present invention is hereinafter detailed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) A (Meth)acrylate Compound Derived from Epoxy Resin

The number average molecular weight of the above-mentioned(meth)acrylate compound (1) derived from epoxy resin is 700 to 3000. Anumber average molecular weight less than 700 weakens adhesive property,while a number average molecular weight more than 3000 deteriorateslight resistance and high temperature high humidity resistance. Thenumber average molecular weight is preferably 1000 to 2500. The numberaverage molecular weight could be measured by a method of gel permeationchromatography (GPC).

The above-mentioned (meth)acrylate compound derived from epoxy resin tobe used includes, for example, epi-bis type epoxy resin derivative(meth)acrylate having a number average molecular weight of 700 to 3000,which involves epi-bis type diglycidyl ether (meth)acrylate such as bisA diglycidyl ether di(meth)acrylate and bis F diglycidyl etherdi(meth)acrylate; an adduct of ε-caprolactone-modified (meth)acrylate toepi-bis type epoxy resin such as an adduct of ε-caprolactone-modified(meth)acrylate to bis A diglycidyl ether (a caprolactone addition molarnumber of 1 to 10) and an adduct of ε-caprolactone-modified(meth)acrylate to bis F diglycidyl ether (a caprolactone addition molarnumber of 1 to 10); a combination of two or more of these; and the like.

Among these, an adduct of ε-caprolactone-modified (meth)acrylate toepi-bis type epoxy resin is preferable. Above all, a (meth)acrylatecompound represented in the following general formula (II) is morepreferable.

In the formula, X denotes a methylene group, an ethylidene group or anisopropylidene group. R denotes a direct bond or a divalent organicgroup. n is an integer of 1 to 10, preferably 2 to 6. Theabove-mentioned divalent organic group may be, for example, a residuederived from divalent carboxylic acid with a carbon number of 2 or moresuch as a group represented in —OOCACO— (wherein A denotes a direct bondor a diester bond residue of divalent carboxylic acid with a carbonnumber of 3 or more). Among the above, the divalent carboxylic acid witha carbon number of 3 or more is not particularly restricted and involvesmalonic acid, succinic acid, adipic acid, maleic acid, itaconic acid,fumaric acid, phthalic acid, terephthalic acid, isophthalic acid,hexahydrophthalic acid, tetrahydrophthalic acid, dimer acid, and thelike.

A compound represented in the above-mentioned general formula (II) canbe obtained, for example, by a method of reactingε-caprolactone-modified (meth)acrylate to which 1 to 10 mol ofε-caprolactone is added with epi-bis type epoxy resin.

The quantity of the above-mentioned (meth)acrylate compound (1) derivedfrom epoxy resin to be mixed is preferably 10 to 80 parts by weight in100 parts by weight of the resin composition, more preferably 30 to 60parts by weight in consideration of the viscosity of the resincomposition to be obtained.

(2) A (Meth)acrylate Monomer Having One or More of Unsaturated EthyleneGroups

The above-mentioned (meth)acrylate monomer having one or more ofunsaturated ethylene groups is not particularly restricted and may beone kind or two or more kinds of monofunctional or polyfunctional(meth)acrylate. Examples of the above-mentioned monofunctional orpolyfunctional (meth)acrylate are hereinafter listed.

Monofunctional (Meth)acrylate

Monofunctional (meth)acrylate is not particularly restricted andinvolves isobutyl (meth)acrylate, tert-butyl (meth)acrylate,phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, tetrahydrofurfuryl(meth)acrylate, isooctyl (meth)acrylate, lauryl (meth)acrylate, stearyl(meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,4-hydroxybutyl (meth)acrylate, 2-methoxyethyl (meth)acrylate,methoxytriethylene glycol (meth)acrylate, 2-ethoxyethyl (meth)acrylate,3-methoxybutyl (meth)acrylate, ethyl Carbitol™ (meth)acrylate,phenylglycidyl ether (meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, tricyclodecane (meth)acrylate, dicyclopentenyl(meth)acrylate, dicyclopentenyloxymethyl (meth)acrylate,dioxolane-modified (meth)acrylate, and the like.

Difunctional (Meth)acrylate

Difunctional (meth)acrylate is not particularly restricted and involves1,4-butanediol di(meth)acrylate, 1,5-pentanediol di(meth)acrylate,1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate,1,12-dodecanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate,(poly)ethylene glycol di(meth)acrylate, (poly)propylene glycoldi(meth)acrylate, 2-n-butyl-2-ethyl-1,3-propanediol di(meth)acrylate,bisphenol A ethylene oxide 1 to 4 mol addition-modifieddi(meth)acrylate, bisphenol A propylene oxide 1 to 4 moladdition-modified di(meth)acrylate, bisphenol F ethylene oxide 1 to 4mol addition-modified di(meth)acrylate, bisphenol F propylene oxide 1 to4 mol addition-modified di(meth)acrylate, tricyclodecanedi(meth)acrylate, tricyclodecane dimethylol di(meth)acrylate,hydroxypivalic neopentyl glycol di(meth)acrylate,ε-caprolactone-modified hydroxypivalic neopentyl glycoldi(meth)acrylate, and the like.

Trifunctional or More (Meth)acrylate

Trifunctional or more (meth)acrylate is not particularly restricted andinvolves tris((meth)acryloxyethyl)isocyanurate, trimethylol propanetri(meth)acrylate, trimethylol propane ethylene oxide 1 to 4 mol addedtri(meth)acrylate, trimethylol propane propylene oxide 1 to 4 mol addedtri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, glycerin-modified tri(meth)acrylate,dipentaerythritol hexa(meth)acrylate, and the like.

Among these, the following are preferable: tricyclodecane(meth)acrylate, tricyclodecane di(meth)acrylate, tricyclodecanedimethylol di(meth)acrylate, dicyclopentenyl (meth)acrylate,dicyclopentenyloxyethyl (meth)acrylate, isobornyl (meth)acrylate,hydroxypivalic neopentyl glycol di(meth)acrylate, trimethylol propanetri(meth)acrylate, trimethylol propane ethylene oxide 1 to 4 mol addedtri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate. Above all, dicyclopentenyl (meth)acrylate andtricyclodecane dimethylol di(meth)acrylate are more preferable due to asuperior curability on a resin surface and an end face of substrates tobe stuck together, which are easily affected by oxygen in the air to behindered from curing.

The quantity of the above-mentioned (meth)acrylate monomer (2) havingone or more of unsaturated ethylene groups to be mixed is preferably 20to 90 parts by weight in 100 parts by weight of the resin composition,more preferably 40 to 70 parts by weight.

(3) A Polymerization Initiator Represented in the General Formula (I)

The above-mentioned polymerization initiator represented in the generalformula (I) is not particularly restricted and involves2,2-dimethoxy-2-phenylacetophenone, benzoylethyl ether, benzoylisobutylether, a combination of two or more kinds of these, and the like.

Also, as required, a compound except the above-mentioned polymerizationinitiator represented in the general formula (I) may be used togethertherewith; for example, one kind or two or more kinds of athioxanthone-based compound such as 2,4-diethylthioxanthone and2-isopropylthioxanthone, an acylphosphine oxide-based compound such as2,4,6-trimethylbenzoyldiphenylphosphine oxide andbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, and thelike.

The quantity of the above-mentioned polymerization initiator representedin the general formula (I) to be mixed is preferably 0.2 to 10 part(s)by weight in 100 parts by weight of the resin composition, morepreferably 1 to 7 part(s) by weight.

In the case of using the compound except the polymerization initiatorrepresented in the general formula (I), a proper quantity thereof can bemixed within a range of no hindrance to the object of the presentinvention.

A tetrazole derivative may be mixed as desired into the ultravioletcuring resin composition for an optical disk of the present invention.The above-mentioned tetrazole derivative is not particularly restrictedand involves 1-phenyl-5-mercaptotetrazole, 5-aminotetrazole,1-methyl-5-mercaptotetrazole, 5-phenyltetrazole,1-(2-dimethylaminoethyl)-5-mercaptotetrazole, a combination of two ormore kinds of these, and the like.

The quantity of the above-mentioned tetrazole derivative to be mixed ispreferably 0.01 to 1 part by weight in 100 parts by weight of the resincomposition, more preferably 0.05 to 0.5 part by weight. The quantity tobe mixed more than 1 part by weight brings the possibility ofdeteriorating storage stability, while the quantity to be mixed lessthan 0.01 part by weight brings the possibility of insufficientlyoffering protective performance toward a reflective film made of silveror silver alloy.

Other additives may be mixed as required into the ultraviolet curingresin composition for an optical disk of the present invention. Theabove-mentioned additives are not particularly restricted and thefollowing may be used in accordance with purposes unless the object ofthe present invention is hindered, for example, a silane coupling agentsuch as alkyl-based, thiol-based, (meth)acrylate-based,isocyanate-based, and epoxy-based; a polymerization inhibitor such asmethoquinone and methyl hydroquinone; an antioxidant such as hinderedphenol, hindered amine, and phosphite; a leveling agent; an antifoamingagent; and the like.

Urethane (meth)acrylate and polyester (meth)acrylate may be mixed intothe ultraviolet curing resin composition for an optical disk of thepresent invention for the purpose of adjusting the properties such asviscosity and hardness unless the object of the present invention ishindered.

The ultraviolet curing resin composition for an optical disk of thepresent invention can be manufactured by mixing and stirring each of theabove-mentioned components at normal temperature to a temperature ofapproximately 80° C., preferably under a reduced pressure, with the useof an organic solvent or without using it.

The ultraviolet curing resin composition for an optical disk of thepresent invention can be used appropriately for the optical disk inwhich an information recording layer is formed by thin films made ofsilver or alloy comprising silver as a major component. It should benoted that the constitution of the alloy comprising silver as a majorcomponent is not particularly restricted in the present specificationand includes, for example, alloy comprising silver slightly containingone kind or two or more kinds of gold, copper, rhodium, palladium,magnesium, nickel, aluminum, and the like. The composition of thepresent invention can be used for the optical disk having thin filmsmade of the above-mentioned components as well as an optical disk havingan information recording layer otherwise constituted such as an opticaldisk in which an information recording layer is formed by thin filmsmade of alloy comprising copper containing silver and thin films notcontaining silver. Also, raw materials of a substrate applied to theoptical disk are not particularly restricted and may be, for example,polycarbonate-based raw material, polyacryl-based raw material,amorphous polyolefin-based raw material, polyvinyl-based raw material,and the like.

EXAMPLES

The present invention is further detailed hereinafter by examples and isnot restricted thereto.

Raw materials used in the after-mentioned examples and comparativeexamples are as follows:

Ebecryl 3708: an adduct of ε-caprolactone-modified acrylate to bis Adiglycidyl ether, manufactured by DAICEL UCB Co., Ltd. (a number averagemolecular weight of 2200, which is measured by a method of GPC with asolvent of tetrahydrofuran, similarly measured hereinafter)

KRM 7856: an adduct of ε-caprolactone-modified acrylate to bis Adiglycidyl ether, manufactured by DAICEL UCB Co., Ltd. (a number averagemolecular weight of 1900)

VR 77: bisphenol A-type epoxy resin diacrylate, manufactured by SHOWAHIGHPOLYMER Co., Ltd. (a number average molecular weight of 800)

KRM 7811: aliphatic urethane diacrylate, manufactured by DAICEL UCB Co.,Ltd. (a number average molecular weight of 3500)

FA-511A: dicyclopentenyl acrylate, manufactured by HITACHI CHEMICAL Co.,Ltd.

HPP-A: hydroxypivalic neopentyl glycol diacrylate, manufactured byKYOEISHA CHEMICAL Co., Ltd.

DCPA: tricyclodecane dimethylol diacrylate, manufactured by KYOEISHACHEMICAL Co., Ltd.

IC 651 (the brand name of IRGACURE 651):2,2-dimethoxy-2-phenylacetophenone, manufactured by CIBA SPECIALTYCHEMICALS Inc.

IC 184 (the brand name of IRGACURE 184): 1-hydroxycyclohexylphenylketone, manufactured by CIBA SPECIALTY CHEMICALS Inc.

IC 907 (the brand name of IRGACURE 907):2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propane-1-one,manufactured by CIBA SPECIALTY CHEMICALS Inc.

L-TPO (the brand name of LUCIRIN TPO):2,4,6-trimethylbenzoyldiphenylphosphine oxide, manufactured by BASF Co.,Ltd.

MMT: 1-methyl-5-mercaptotetrazole, manufactured by TOYO KASEI KOGYO Co.,Ltd.

PMT: 1-phenyl-5-mercaptotetrazole, manufactured by TOYO KASEI KOGYO Co.,Ltd.

IBXA: isobornyl acrylate, manufactured by OSAKA ORGANIC CHEMICALINDUSTRY Ltd.

Examples 1 to 9 and Comparative Examples 1 to 4

Each of the raw materials was measured out into a flask in accordancewith the mixtures shown in Table 1 so as to be stirred at a temperatureof 60° C. under a reduced pressure for 1 hour and be made intoultraviolet curing resin as uniform transparent liquid at each of themixtures. It should be noted that the mixtures shown in Table 1 weredenoted part(s) by weight.

Two sheets of polycarbonate substrates having a thickness of 0.6 mm wereused for optical disks; one of them was a polycarbonate substrate onwhich a metallic reflective film having a thickness of approximately 15nm was formed by sputtering silver alloy (the brand name of TTP32A,manufactured by Target Technology Co.), and the other was apolycarbonate substrate on which a metallic reflective film having athickness of 50 nm was formed by sputtering aluminum.

Approximately 2 g of the ultraviolet curing resin prepared above wasapplied between the above-mentioned two substrates of disks and extendedby a spin coater at a rate of 3700 rpm for 3 seconds, so as to obtain auniform adhesive layer having a thickness of approximately 40 μm.

Thereafter, the adhesive layer was irradiated with ultraviolet rayhaving an intensity of 600 mJ/cm² by a metal halide lamp to be cured,thereby obtaining a disk sample which consists of two substrates boundedtogether.

Evaluating Methods

The obtained disk samples which consists of two substrates boundedtogether were evaluated by the following methods. The results are showntogether in Table 1.

Evaluation of Adhesive Property

The side of the adhesive layer of the disk samples was cut with a cutterknife to peel away the two sheets of substrates, so that adhesiveproperty was evaluated by a peeling mode at the moment. The standard ofevaluation was as follows:

◯: Adhesive strength was high and the metallic reflective film waspeeled off the substrate.

Δ: The metallic reflective film was peeled off the adhesive layer withdifficulty, though.

x: Adhesive force was weak and the metallic reflective film was easilypeeled off the adhesive layer.

Evaluation of High Temperature High Humidity Resistance

The optical disks remained untouched under the conditions of anatmosphere having a temperature of 85° C. and a relative humidity of 85%for 240 hours, so that defects such as corrosion and discoloration wereobserved by an optical microscope. The standard of evaluation was asfollows:

◯: No change occurred as compared with the initial state.

x: Defects such as discoloration and pinhole occurred as compared withthe initial state.

Evaluation of Light Resistance

The optical disks were irradiated by a sunshine weatherometer (a xenonlamp source) for 100 hours, so that the quality of light resistance wasdetermined by the rate of change in the reflectance of the metallicreflective film before and after being irradiated. The standard ofevaluation was as follows:

A: The rate of decreases in the reflectance was less than 5%.

B: The rate of decreases in the reflectance was 5% to less than 10%.

C: The rate of decreases in the reflectance was 10% to less than 20%.

D: The rate of decreases in the reflectance was 20% or more.

TABLE 1 Comparative Examples Examples 1 2 3 4 5 6 7 8 9 1 2 3 4Ebecryl3708 50 50 50 — 50 — — 50 50 — 50 50 — KRM7856 — — — 50 — 50 — —— — — — — VR77 — — — — — — 50 — — — — — 50 KRM7811 — — — — — — — — — 50— — — FA-511A 50 50 50 50 30 — 50 — 50 50 — 50 — HPP-A — — — — — 50 — —— — 50 — — DCPA — — — — 20 — — — — — — — 50 IBXA — — — — — — — 50 — — —— — IC651  3  3  3  3  3  3  3  3  3  3 — — — IC184 — — — — — — — — — — 3 —  3 IC907 — — — — — — — — — — —  3 — L-TPO — — — — — — — —   0.2 — —— — MMT —   0.2 —   0.2   0.2   0.2 —   0.2 — — — — — PMT — —   0.2 — —— — — — — — — — Adhesive ∘ ∘ ∘ ∘ ∘ ∘ Δ ∘ ∘ ∘ ∘ ∘ x Property High ∘ ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ x ∘ ∘ ∘ Temperature High Humidity Resistance Light B A A A A AB A B D D D D Resistance

It was understood from the examples that the composition of the presentinvention sufficiently fulfilled all performances of adhesive property,high temperature high humidity resistance, and light resistance. Incontrast, the comparative examples not employing the constitution of thepresent invention exhibited an extremely low light resistance among theperformances. In addition, Comparative Example 4 employing the mixturein conventional techniques exhibited extremely low results with regardto both of adhesive property and light resistance.

INDUSTRIAL APPLICABILITY

An ultraviolet curing resin composition for an optical disk of thepresent invention is constituted as described above, so as to vastlyimprove the light resistance of reflective thin films made of silver orsilver alloy as compared with conventional techniques and effectivelyprotect them against corroding. Also, an adhesive layer is flexible andoffers a high adhesive strength, leading to a contribution to animprovement in the mechanical strength of optical disks.

What is claimed is:
 1. An ultraviolet curing resin composition for anoptical disk comprising: (1) an adduct of ε-caprolactone-modified(meth)acrylate to epi-bis type epoxy resin having a number averagemolecular weight of 700 to 3000 derived from epoxy resin; (2) a(meth)acrylate monomer having one or more of unsaturated ethylenegroups; and (3) a polymerization initiator represented in the followinggeneral formula (I)

in the formula, R1 denotes an alkyl group with a carbon number of 1 to4, R² denotes a hydrogen atom or an alkyl group with a carbon number of1 to 4, and R3 denotes a phenyl group or an alkyl group with a carbonnumber of 1 to
 4. 2. The ultraviolet curing resin composition for anoptical disk according to claim 1, wherein a tetrazole derivative isadditionally mixed.
 3. The ultraviolet curing resin composition for anoptical disk according to claim 1, wherein said adduct ofε-caprolactone-modified (meth)acrylate to epi-bis type epoxy resin (1)is a compound represented in the following general formula (II)

in the formula, X denotes a methylene group, an ethylidene group or anisopropylidene group, R denotes a direct bond or a divalent organicgroup, and n is an integer of 1 to
 10. 4. The ultraviolet curing resincomposition for an optical disk according to claim 1, wherein said(meth)acrylate monomer (2) having one or more of unsaturated ethylenegroups is at least one kind selected from the group consisting ofdicyclopentenyl (meth)acrylate, tricyclodecane dimethyloldi(meth)acrylate, and hydroxypivalic neopentyl glycol di(meth)acrylate.5. The ultraviolet curing resin composition for an optical diskaccording to claim 1, wherein said polymerization initiator (3)represented in the general formula (I) is at least one kind selectedfrom the group consisting of 2,2-dimethoxy-2-phenylacetophenone,benzoylethyl ether, and benzoylisobutyl ether.
 6. The ultraviolet curingresin composition for an optical disk according to claim 2, wherein thetetrazole derivative is at least one kind selected from the groupconsisting of 1-phenyl-5-mercaptotetrazole, 5-aminotetrazole,1-methyl-5-mercaptotetrazole, 5-phenyltetrazole, and1-(2-dimethylaminoethyl)-5-mercaptotetrazole.
 7. The ultraviolet curingresin composition for an optical disk according to claim 2, wherein aquantity of the tetrazole derivative to be mixed is 0.01 to 1 part byweight in 100 parts by weight of the resin composition.
 8. Theultraviolet curing resin composition for an optical disk according toclaim 1, wherein the ultraviolet curing resin composition for an opticaldisk is such that an information recording layer is formed by a thinfilm made of silver or alloy comprising silver as a major component.